Effect of the Secondary Reduction on the Enantioselectivity and Function of Additives in the Chiral Oxazaborolidine‐Catalyzed Asymmetric Borane Reduction of Ketones

Abstract

The secondary reduction in the direct and oxazaborolidine-catalyzed asymmetric borane reduction of ketones was investigated by the use of GC/MS tracing titration and control experiments. The results indicate that the secondary reduction affects the enantioselectivity only in noncoordinated solvents at low temperature and not under the usual catalytic reduction conditions because the intermediate alkoxy-A C H T U N G T R E N N U N G borane is unstable and quickly converts to borane and dialkoxyborane. The function of an alcohol additive in the asymmetric borane reduction of ketones is to consume excess borane in the reduction system thus inhibiting noncatalytic reduction, which leads to increased enantioselectivity in the catalytic reduction.1. Introduction. -Chiral oxazaborolidine (OAB)-catalyzed borane reduction of prochiral ketones to chiral secondary alcohols is considered to be one of the most important reactions in asymmetric syntheses (for recent reviews, see [1] [9] [10] on the enantioselectivity of the asymmetric reduction, these effects are still unclear. However, it is highly desirable to understand all factors which affect the enantioselectivity in order to apply the asymmetric reaction effectively. Herein, we wish to present our investigation about the effect of the secondary reduction on the enantioselectivity and about the function of additives in the asymmetric borane reduction of ketones.